It is well known to add biocides to thermoplastic resin compositions to protect articles formed from such compositions against microbial degradation. For processing reasons, it is known to provide biocides as concentrates, such as liquid concentrates as taught, for example, in U.S. Pat. No. 4,758,609, or solid concentrates as taught, for example, in U.S. Pat. No. 4,086,297, the teachings of each of which are incorporated herein by reference. Although a number of biocides have been suggested for use in thermoplastic resins, including isothiazolinones, the standard anti-microbial agent in the industry has been and remains 10,10'-oxybisphenoxarsine (OBPA). While OBPA has proven to be a very effective biocide for use in the plastics industry, there is perceived an eventual need to replace OBPA due to its heavy metal (arsenic) content.
Isothiazolinone compounds are one class of biocides considered to be effective replacements for OBPA and similar heavy metal-containing biocides. At room temperature, some isothiazolinone compounds are in solid form and therefore difficult to handle. Such solid isothiazolinones may be more easily handled if dissolved in a liquid carrier, such as a plasticizer. Even if the isothiazolinone is in liquid form at room temperature, it is often desirable to dilute the isothiazolinone with a carrier to minimize mixing problems. Accordingly, liquid isothiazolinone, concentrates, have been prepared, as taught, for example, in U.S. Pat. No. 5,028,619.
The carrier for a liquid concentrate preferably is a liquid that also has a function in the end-use resin composition to which the concentrate is added so as to provide anti-microbial properties, and typically in liquid concentrates the carrier is a plasticizer for the end-use resin composition. A problem with concentrates of isothiazolinone-in-plasticizer concentrates wherein the isothiazolinone is present in amounts of greater than about 4 wt %, particularly in amounts of about 10 wt % or greater, is that in sub-freezing conditions which the concentrate may encounter if shipped or stored in winter, the isothiazolinone compounds tend to crystallize from solution or the entire concentrate solution tends to freeze. This is a significant inconvenience for the plastic processor who must wait for the isothiazolinone to redissolve or the concentrate to thaw and then make sure that the concentrate is homogeneous.
It is a general object of the present invention to provide isothiazolinone concentrates which are stable to crystallization and/or freezing in winter shipping and storage conditions.